In recent years, in a battery driving equipment, in particular, in a portable information-processing equipment such as a disk player, a cellular phone and the like, miniaturization of equipment and long-term battery driving are being strongly demanded. And, one of key points therefor is power-consumption lowering technologies. With low power consumption, long-term driving and weight lightening of a battery can be attained at the same time. Therefore, it is strongly demanded that a motor to be used in the equipment is designed to contribute to such power-consumption lowering.
The present applicant has disclosed in Japanese Patent Application Laid-Open No. 9-9554 an example of a slender cylindrical coreless motor (hereinafter, referred to as motor) directed to use in portable equipments. FIG. 3 shows the configuration thereof. As shown in the drawing, this motor is configured by comprising a frame 51 in shape of a slender cylinder, a magnet 53 inside the frame 51 and having a cylindrical shape with its end portion fixed to the aforementioned frame, a rotor assembly body 54 having a cylindrical coil encircling the magnet 53, a shaft 55 having an end fixed at the rotor assembly body 54, an outer bearing 56 which is disposed at an end portion of the frame 51 and bears the outer protruding part of the shaft 55, an inner bearing 57 which bears a part of the shaft 55 fixed to the rotor assembly body 54, a commutator 58 supplying electricity to the rotor assembly body 54, a brush 59, and a load dispatching terminal 60. And, a magnetic circuit is formed by the magnetized magnet 53 and the frame 51 encircling the magnetized magnet 53. On the other hand, load dispatching is implemented from the dispatching terminal 60 through the brush commutator to the rotor assembly body 54 which is then magnetized, driven and rotated, and outputting is implemented to the outside of the motor by the shaft 55. A configuration where a shaft penetrates through the center of a cylindrical magnet has been conventionally known. The above-described motor is suitable to realize a motor with a small diameter because the shaft 55, the outer bearing 56, the inner bearing 57 in which an inner bearing metal 63 is housed in a case 62, and the inner substance magnet 53 are disposed in different positions toward the axis so as not to interrupt with each other.
However, from the view point of furthering the power-consumption lowering, in the case where a large side pressure load is imposed on the output axis, with such a structure as in this example that the spacious distance between both bearings could not be easily taken, the problem that pivot loss got greater which obstructed power-consumption lowering came into existence.
On the other hand, a proposal trying to realize a small-diameter motor being left with a cylindrical magnet is found in Japanese Patent Laid-Open No. 10-83622 specification. FIG. 5 shows the structure thereof. This motor comprises a pipe 72 inside a frame 71 with its end portion fixed to the aforementioned frame 71, a cylindrical magnet 73 having been disposed at outside around the pipe 72, a cylindrical rotor assembly body 74 encircling the magnet 73, a shaft 75 having an end fixed at the rotor assembly body 74, an outer bearing 76 which is disposed at an end portion of the frame 71 and bears the outer protruding party of the shaft 75, and an inner bearing 77 which bears the fixed party of the rotor assembly body 74 of the shaft 75. And, the pipe 72 protrudes from the end surface of the magnet 73 and extends, while the inner bearing 77 is provided with a dent portion 77a having inner diameter as well as length fitting the outer diameter of the aforementioned extended protruding portion 72a, which are fixed to each other at this portion. This structure is said to make a motor with smaller outer diameter realizable without any interruption against the magnet 73 and the inner bearing 77.
However, from the view point of further proceeding with power-consumption lowering, such a method of attaching the inner bearing as in this example cannot always be designated as the best method. The reason is that within a limited length of the rotor assembly body, length of the magnet has been sacrificed to create the above-described fixed space. Reduction in the magnetic flux generated in a magnet directly results in reduction in the torque constants, that is an increase in the power consumed. In addition, in this configuration, the bearing is held at the cylindrical portion of the pipe head, but as the long pipe protruding from the magnet is too long compared with the small diameter, accuracy of the shape thereof can hardly be maintained. Accordingly, the construction that the inner bearing is attached depending on the outer diameter of the protruding portion of the pipe, and further the magnet is attached makes it difficult to maintain concentric nature of the inner bearing as well as the outer diameter of the magnet. Therefore, the air gap between the rotor assembly body and the magnet can not help being made wide. This is also another problem from the view point of further proceeding with power-consumption lowering.